The narrative gears of Wormwood click into place in a tremendously satisfying way with “The Forbidden Threshold,” which drills deep into a single line of inquiry surrounding Frank Olson’s death while also layering on a deeper context about the U.S. government’s culpability in biological warfare. The whole episode revolves around that 13th-story room in the Statler Hotel, which Eric Olson returns to many years later to commune with the spirit of his father, not to uncover any new evidence. After so much buildup, the room is remarkable for its sheer ordinariness: If Olson hadn’t fallen from it, none of us would be here.

It’s evident that the success of any individual portion of Wormwood is largely dependent on how well Eric Olson is able to make his points during his extended interviews. The series lives and dies by his obsessions: If he’s hitting the CIA hard on an inadequate or contradictory explanation, Errol Morris will find a way to turn that into compelling visuals. If he’s just rambling, Morris has to pull off more sleight of hand to make up for it. Here, Olson’s got a lot of grist to chew on regarding the suspicious behavior of his father’s companion, Robert Lashbrook (played by Christian Camargo), who was in the hotel room with him but did not stop him from jumping out of the window, and made a few highly suspicious phone calls in the aftermath, neither of them to the police. (The reenactments imply Lashbrook had deliberately locked himself in the bathroom during the incident, but also that he felt kinda bad about it?) Olson further notes that, had Frank indeed dived out the window of his own volition, “that would have been an amazing stroke of luck for them,” because he was removing himself from the CIA’s list of concerns just at the moment when he became a liability.

These are, alas, merely hunches, and indulging hunches is entertaining but ultimately a bit empty. Morris tends to structure his films around single interview subjects, which turns his work into either a dictation exercise or a fencing battle between him and his interviewee. Wormwood is the former, and the fact that we’ve made it halfway through the series without its reliance on Eric becoming overbearing is a testament to how well Morris teases out the different threads of his story. Pulling on the strings of Frank Olson’s comment to Lashbrook shortly before his death — “Can’t I just disappear?” — he reasons that Olson could have merely been asking to leave the scene, not expressing suicidal thoughts. And if Lashbrook had reason to believe it was the latter, anyway, why didn’t he tell the hotel desk so they could move Olson to a lower floor? Again, hunches. They sound damning everywhere but in a court of law.

If there’s one hunch that really gets the juices flowing, though, it’s the idea that the CIA wanted Olson dead because he knew about the use of biological weapons during the Korean War. The history lesson in “The Forbidden Threshold” gives us a primer on the popularization of the term “brainwashing,” which, for the U.S. government’s purposes, was a convenient way to discount Chinese-extracted confessions from captured American soldiers about germ warfare. By claiming the soldiers were simply saying whatever the Chinese were controlling them to say, by claiming that they were “brainwashed,” those confessions would immediately seem suspect.

Morris carefully tiptoes around the (still hotly debated) matter of the germ warfare itself, which so far exists outside the scope of this investigation. His main point in alluding to it in “The Forbidden Threshold” seems to be the rhetorical aspect of it all: You don’t have to deal with the truth if you can demonstrate the truth-teller was under psychological duress. There’s an obvious thematic link here in the CIA’s own treatment of its MKUltra subjects, in the many possible explanations for Olson’s behavior, and in the general obsession with finding ways to dodge obvious truths that Morris returns to again and again in his work.

Wormwood could do with more flights of fancy like this to break up Eric Olson’s accountings, but at the halfway mark, it’s left us with plenty of tantalizing ideas to chew on. The rules of plausible deniability have changed, and the government will not be cornered when it tells falsehoods, even for a case as outlandish as Olson’s. More psychological duress is sure to follow if the evidence keeps mounting.

Burning Questions

• Lashbrook: Friend or foe? And how much of his prior knowledge of assassination techniques would have come in handy in that hotel room?

• Are we ever going to find out the truth of what Frank supposedly knew, and just how dangerous it was?

• Will Morris, who can grill with the best of them, ever turn the tables on Eric Olson and try to poke holes in his version of events? Or will he continue siding with the skeptic?